1. Field of the Invention
The present invention relates to a method for recovering lost ethyl acrylate potential (EAP) and, more particularly, to the recovery of acrylic acid and/or ethyl acrylate from spent black acid obtained in the production of ethyl acrylate by reaction of ethylene, acrylic acid and sulfuric acid.
2. Description of the Prior Art
Ethylenically unsaturated organic compounds, such as acrylic acid, methacrylic acid, methacrylic esters, acrylate esters, and the like, are widely used in the production of homopolymers and copolymers. These homopolymers and copolymers, produced readily through the polymerization of the available double bonds of the organic compounds, are widely used in paints, coatings, lacquers and the like. The olefinic activity of the ethylenically unsaturated organic compounds, makes the polymerized products highly useful for many purposes.
Processes for the production of ethyl acrylate by the interaction of acrylic acid with ethylene in the presence of a sulfuric acid catalyst are well known. As examples see U.S. Pat. No. 3,703,539, issued Nov. 21, 1972, to DiLiddo; U.S. Pat. No. 3,539,621, issued Nov. 10, 1970, to Cipollone et al.; U.S. Pat. No. 3,894,076, issued Jul. 8, 1975, to Van Duyne et al.; and U.S. Pat. No. 4,490,553, issued Dec. 25, 1984, to Chase et al., all of which are incorporated by reference. In these and other references, the reaction is believed to involve the formation of intermediate sulfates from the reaction of ethylene with sulfuric acid. These sulfates further react with acrylic acid to form ethyl acrylate and product mixtures of unreacted ethylene, acrylic acid and sulfuric acid residue which must be recycled to the reactor tower. The product mixture is thus sent to a distillation train comprising a recovery distillation tower, a light ends distillation tower and finishing tower, all of which are conventional. In the recovery tower, the mixture is distilled under vacuum to obtain light ends of crude ethyl acrylate which are passed to a light ends distillation column where a partially purified ethyl acrylate bottoms product is sent to a finishing distillation tower for recovery of substantially pure ethyl acrylate. The bottoms product from the recovery distillation tower is a sulfuric acid residue or black acid stream containing free sulfuric acid, ethyl hydrogen sulfate, diethyl sulfate, lactone polyester, acrylic acid and ethyl acrylate. The sulfuric acid residue or black acid stream is removed as bottoms residue from the recovery distillation tower and then returned to the reactor tower to repeat the cycle. During the ethylene-acrylic acid reaction, black acid builds up in viscosity and to maintain proper viscosity values, which allow favorable use of process equipment and not retard reaction rates, some of the black acid is periodically purged from the system, usually about 1 to 5 wt. %, and new reactants are added. The purged or blow down material, which is rich in ethyl acrylate potential, is reprocessed for sulfur recovery. During the reprocessing of the spent black acid, the ethyl acrylate potential (EAP) is thus lost. Improvement in ethyl acrylate yields would be substantially increased if the EAP could be recovered.